1. Field of the Invention
The present invention relates to an imaging apparatus, a control method thereof, and an imaging system.
2. Description of the Related Art
There is a digital camera with a zoom display function as an imaging apparatus having an electronic zoom function (Japanese Patent Laid-Open No. 11-196335). This apparatus uses a frame memory as an image display memory. This apparatus also includes an image display means capable of displaying a moving image, a means for designating a desired image, and an enlarged image display means (electronic zoom function) for enlarging the designated image at a predetermined magnification and displaying the enlarged image on an image display device. This allows zooming in a moving image, still image, or reproduced image on a liquid crystal display device during imaging.
Another imaging apparatus implements an electronic zoom function by changing the sampling frequency of an input image signal (Japanese Patent Laid-Open No. 9-163236). This apparatus includes a charge-coupled image sensor, analog signal processing unit, A/D conversion unit, image memory serving as a frame buffer, digital signal processing unit, D/A conversion unit, monitor, variable sampling clock generation unit, system control unit, and magnification setting unit.
There is still another imaging apparatus that eliminates the exposure period difference between the upper and lower parts of one frame (Japanese Patent Laid-Open No. 2005-94142). This apparatus includes a register that holds part of frame image data output from an image sensor unit, and a control unit that performs a process of writing electronic zoom image data in the register and a process of reading out the held image data of at least one row at a predetermined frame rate. The apparatus also includes a resolution conversion unit that interpolates the readout image data based on the electronic zoom magnification to convert it to the size of image data of one frame.
The imaging apparatuses described in Japanese Patent Laid-Open Nos. 11-196335 and 9-163236 must incorporate a frame memory with a large capacity to keep a predetermined frame rate upon electronic zoom. This results in an increase in power consumption and in the cost and size of the imaging apparatuses. It is particularly difficult to use them for mobile applications. In these imaging apparatuses, the horizontal scan period changes depending on the driving method. Since the exposure period of each frame is an integer multiple of the horizontal scan period, it is difficult to maintain a predetermined exposure period independently of the driving method. When the driving method changes, the exposure period of a frame read out immediately after the change is different from that of a frame before the change. That is, the preceding and succeeding frames have a difference in brightness. Additionally, even in the frame read out immediately after the driving method changes, the exposure period changes between the lines. Hence, the brightness changes in the vertical direction.
The imaging apparatus described in Japanese Patent Laid-Open No. 2005-94142 is a proposal to solve these problems. However, since the readout time of one frame changes between driving modes (readout modes), the degree of distortion of the rolling shutter abruptly changes upon, for example, switching the electronic zoom magnification, resulting in a sense of incongruity of the user.